A U-shaped association between the systemic immune-inflammation index and refractory Mycoplasma pneumoniae pneumonia in children: A retrospective cohort study

OBJECTIVES: This study aimed to explore the association between the SII and RMPP incidence, focusing the dose-response relationship to provide new insights into RMPP pathogenesis. METHODS: This study included 1,251 children with MPP who were hospitalized for the first time at the Respiratory Department of Wuxi Children's Hospital between December 2022 and December 2023. Data collected from the patients' medical records included demographics, clinical features, laboratory results, and imaging findings. A multivariate logistic regression model was used to assess the relationship between the SII and RMPP incidence. Generalized additive models (GAMs) and smoothing curve fitting were applied to explore the potential nonlinear association between the SII and RMPP incidence. The inflection point of the SII was determined using recursive algorithms, and two separate piecewise logistic regression models were subsequently constructed on either side of this inflection point. RESULTS: According to the multivariate logistic regression analysis, after adjusting for confounding factors, LogSII was significantly associated with an increased RMPP risk (OR = 1.45, 95% CI: 1.15-1.83, P = 0.002). Curve fitting analysis revealed a nonlinear U-shaped relationship between LogSII and RMPP incidence. Piecewise binary logistic regression analysis revealed an inflection point at LogSII = 6.31, with RMPP risk significantly increasing when LogSII exceeded 6.31 (OR = 2.30, 95% CI: 1.57-3.38, P < 0.001). CONCLUSION: This study revealed a nonlinear, U-shaped association between LogSII and RMPP incidence, with the risk rising substantially when LogSII exceeded 6.31. However, given the retrospective single-center design, external validation is needed before SII can be applied as a predictive tool in broader clinical settings.